MP8007 ― IEEE 802.3AF PD WITH 13W FLYBACK / BUCK CONVERTER

MP8007 Rev.1.0

www.MonolithicPower.com

14

6/27/2016

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OPERATION

Compared with IEEE802.3af, the IEE802.3at

standard establishes a higher power allocation

for

Power-over-Ethernet

while

maintaining

backwards compatibility with the existing

IEEE802.3af

systems.

Power

Sourcing

Equipments (PSE) and Powered Devices (PD)

are distinguished as Type-1 complying with the

IEEE

802.3af

power

levels,

or

Type-2

complying with the IEEE 802.3at power levels.

IEEE802.3af/at standard establishes a method

of communication between PD and PSE with

detection, classification and mark event.

The MP8007 is one integrated PoE solution

with IEEE 802.3af PD interface and 13W DCDC

converter. The PD interface also has 802.3at

function, but DCDC converter only support 13

W output, so MP8007 is only used for 802.3af

power design. Along with the PSE it operates

as a safety device to supply voltage only when

the power sourcing equipment recognizes a

unique, tightly specified resistance at the end of

an unknown length of Ethernet cable. After

powered from PSE, the MP8007 will regulate

the output voltage based on application with

isolated or non-isolated topology. Figure 1

shows the function diagram of this device, and

Figure 2 shows typical PD interface power

operation sequence.

Figure 2: PD Interface Operation Description

Detection

The RDET connected between DET and VDD pin

is presented as a load to the PSE in the

Detection Mode, when the PSE applies two

“safe” voltages between 2.7V to 10.1V while

measuring the change in current drawn in order

to determine the load resistance. 24.9kΩ(1%)

resistor between VDD and DET pins is

recommended to present one correct signature,

and the valid signature resistance seen from

power interface (PI) is between 23.7kΩ and

26.3kΩ.

The detection resistance seen from PI is the

result of the input bridge resistance in series

with the VDD loading. The input bridge

resistance is partially cancelled by MP8007

effective leakage resistance during detection.

Classification

The classification mode can specify to the PSE

the expected load range of the device under

power, so that the PSE can intelligently

distribute power to as many loads as it can

within its maximum current capability. The

classification mode is active between 14.5V and